ZS: Spamming ammo crates, or standing on hem to prevent people from using them, regardless of what is inside them, should not be allowed. I know many VIPS, and admins, have been [rightfully] anal the few times one has been on and we played on a map when this was a problem, but it'd be easier to control if it were a rule as IMO it is damn annoying, and could even be construed as aiding the zombie team in a way I doubt many people would find reasonable on the part of a human teammate.
Ammo crates are optional, not vital to the gameplay and I personally never had problems with people spamming or standing on top of it, I even do it myself.
It's actually even better when someone does this because then those people who want to get more ammo/nades that they'll ever need actually may think about focusing on the main objective or at least to share the ammo with others, and I myself have never seen anyone who would stand on the ammo crate for the entire time just to be dick, he either get's bored or just simply gets killed by the zombies.